Annular tensile members, commonly referred to as tire bead cores, are designed to securely hold the tire on the rim during use.
A tire rim generally has a rim flange and a bead seat specifically designed to hold the bead portion of the tire in place.
These rims are designed to specific dimensions and tolerances as prescribed by various industry associations. In the United States the Tire & Rim Association sets all rim standards. In Europe, the European Tire and Rim Association sets the rim standards. In Japan the J.P.T.O. sets these standards. For the most part, these rim standards globally insure proper tire fits can be reliably designed. This standardization enables tire makers to design tire beads that safely can be mounted and retained on the rims.
The tire beads provide a radially inner portion between the bead core and the rim that is radially compressed and as this portion is compressed the bead core is placed in tension. Radial compression occurs as the tire is mounted on a tapered rim seat by the action of the internal pressure of the tire pushing the bead axially outwardly toward the vertical bead flange. The tapper on rims typically are about 5° or in the case of commercial truck tires as much as 15°.
What actually makes the bead retaining forces achieve very high values is the bead core. Typically the bead cores are made of one or more steel wires wound in an annular configuration to form a bead bundle in the shape of a circular hoop of any variety of cross-sections. Some bead cores are circular in cross-section, others are square, rectangular, hexagonal or variations of those shapes.
In the past, attempts have been made to make synthetic beads or non-ferrous type bead cores. These bead cores have been limited to use in toys or bicycles. The use of synthetic bead cores achieve a light weight structure but generally at the expense of lower tensile strength or bead wire fretting.
Recently in several Japanese patent publications, the use of a combination of steel wire and aramid cores has been proposed as a lightweight bead core. Yokohma Rubber Company, LTD, in JP4078703, suggests using an arimid fiber drawn in a non-twisted state as a core member which is wound by a steel wire in a spirally wound state on the outside of the arimid core. The result is a circular bead core having a lighter weight than an all steel bead core of similar cross section.
Similarly Sumitomo Rubber Ind., LTD in JP4183614 and Toyo Tire in JP7096720 also use a combination of aramid fiber and steel wire to make a bead core. The Toyo design requires at least a first layer on the core inside diameter being of steel wire. The subsequent layers can be made of aramid fiber cords. This insures the bead does not experience a lowering of lifting force when compared to an all-synthetic bead core. In the Sumitomo concept alternating layers of Aramid cords and steel wires are used. In one configuration, the steel wire and aramid cords are arranged in vertical or radial layers and in another embodiment the aramid and steel are layered horizontally to make a bead core of square or rectangular cross-section.
In each case, care is taken to insure the synthetic fiber is used in a non-twisted configuration. The primary issue with synthetic cords is that when provided in a cable that has the cords twisted there is created several problems. The first is called creep under load. The synthetic cables or cords will stretch under load and as the plastic flows the restraining force actually will lower with time, accordingly the use of steel in a radially innermost layer is essential if bead retention forces are to be reliably constant. A second problem with such beads is a pheoromina called fretting. Aramid, in particular, and many other plastics have a condition where small brittle fractures occur if the cords are placed in compression. Cabling such cords actually increase the likelihood of creating these fractures. Generally, aramid cords are typically used in the carcass plies and because repeated exposure to compression stresses can create cord breakage absent special design considerations. In a bead core, almost all of the loads are in tension except when the bead is helically or spirally wound. In those cases the cords work against each other creating small bending forces, which over time result in minute abrading friction of the adjacent cords. This gives rise to a phenomena referred to as fretting.
The lightweight cable beads show promise in small, lightly loaded tires for bicycles but heretofore have not been considered practical in very heavy-duty load conditions.
In radial airplane tires where lightweight issues are very important. The use of steel bead cores has been the practice. The tires are inflated to about 200 psi (14.1 kg/cm2), and can experience impact loads of 50,000 Ibf (22,680 kg) or greater.
In such tires, tests are conducted to insure adequate safely margins exist. Typically the tires annular tensile members are designed to exceed the strength of the tire's carcass and belt structure. Hydraulic burst test are conducted to failure where water is injected into the tire until the tire fails at some very high pressure. Typically the beads survive these tests with the failure mode occurring in the belts or the plies.
In large off-road tires used in earthmoving equipment. The tire's beads are constructed of steel wire formed in large bundles. The tires operated at very high pressures, typically 100 psi and the bead bundles may measure greater than an inch (2.54 cm) across in cross-section thickness and comprised of hundreds of steel wires.
Similarly commercial truck tires use all steel bead cores. These tires run at about 95 psi or greater and must carry very large loads. In each case the use of all steel bead cores has been the accepted practice.
While lightweight tires are generally understood to run cooler, it has been generally understood that those benefits are derived from a reduction of carcass rubber or tread rubber and not by a reduction in the weight of the bead core.
For these reasons the interest in lightweight bead cores for heavy duty tires has received little attention. Only in aircraft tires has the tire weight issue been considered sufficiently important.
The present invention is directed to a lightweight bead core for heavy-duty tires. The invention was first formulated for an advanced lightweight radial aircraft tire. The analysis has shown that the concept is so cost effective and durable that it can be used in almost any heavy-duty tire application.
The object of the invention was to provide a lightweight, yet high strength bead core for severe service applications such as aircraft tires.
A further object of the invention was to provide a low cost bead core that was competitively priced relative to the all steel bead cores.
These features as well as others are described in the invention described hereinafter.